JP2010256711A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which forcibly ejects, on a preferential basis, excessively charged toner on a toner carrier to an image carrier side, and thereby, which effectively allows a refresh process to function without increasing the toner consumption amount than before. <P>SOLUTION: A toner ejection pattern P, formed on a photoreceptor drum 14 when performing the refresh process, is obtained by repeating development of a line image L, which has a predetermined inclination to a main scan direction, at predetermined intervals over a width H of a development region of a developing roller 35. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using a photosensitive drum and a developing device, and more particularly to an image forming device capable of executing a refresh process for refreshing toner on a developing roller during non-image formation.

  In the conventional image forming apparatus, when the image formation is repeated, particularly when the printing rate on the image (the ratio of the printed area to the area where the image can be formed (paper area); the same applies hereinafter) is low. Since there is little toner used for development by flying from the developing roller (toner carrier) to the photosensitive drum (electrostatic latent image carrier), the toner particles in the developing device are hardly replaced, and the toner is excessively charged. Image density reduction and fogging may occur. In particular, when it is necessary to deal with an image with a high printing rate such as a photograph or a graphic image and an image with a low printing rate such as only characters or logo marks, the variation in the printing rate also increases.

  In such a case, a large amount of toner is ejected from the developing roller to the photosensitive drum side by printing a pattern having a high original printing ratio such as solid, and the toner is transferred to a recording medium to consume the toner. However, if the solid pattern is left unprinted for a long period of time, the toner particles are fixed on the surface of the developing roller due to the influence of humidity and the like without being consumed. Sometimes it doesn't.

  Therefore, the toner surface has been improved so that the charge control ability of the toner is stabilized by optimizing the surface shape, material, or external additive of the toner. At present, it has not yet been surely prevented.

  In order to solve the above problems, various methods for forcibly consuming the toner on the developing roller have been proposed. For example, Patent Document 1 calculates an average printing rate from the number of printed sheets and a printing rate, and calculates the average printing. An image forming apparatus is disclosed in which when the rate falls below a predetermined value, the toner on the developing roller is forcibly discharged to the photosensitive member side to be refreshed by applying an AC bias to the developing roller while superimposing the DC bias. Further, Patent Document 2 proposes an image forming apparatus that forcibly consumes toner on the developing roller in a state where rotation of the photosensitive member and the developing roller is stopped when the average printing rate falls below a predetermined value. Yes.

  In the image forming apparatuses of Patent Documents 1 and 2, it is necessary to discharge the toner for one or more rotations of the developing roller, although the image density reduction and fogging can be suppressed by executing the refresh process. For this reason, the amount of wasteful toner other than printing is increased, and the toner discharged on the photosensitive member is liable to adhere to the transfer roller, causing the recording medium to become dirty.

  Therefore, Patent Documents 3 and 4 disclose a method for efficiently discharging deteriorated toner by using a non-solid pattern such as a staggered pattern, a halftone dot pattern, or a line pattern as a toner ejection pattern.

JP 2000-310909 A JP 2000-330379 A JP 2000-206770 A JP 2009-53582 A

  According to the methods of Patent Documents 3 and 4, the toner on the developing roller can be efficiently consumed by so-called edge enhancement in which the potential difference in the edge area of the toner ejection pattern is larger than that in the solid area. However, with the recent demands for improved image forming efficiency and lower running costs of the device, the consumption of toner other than printing is further reduced, and adhesion to the transfer roller is suppressed to ensure that the back side of the recording medium is stained. Development of a method to prevent this has been desired.

  In view of the above-described problems, the present invention effectively functions the refresh process without increasing the amount of toner consumption compared with the prior art by preferentially forcibly discharging the overcharged toner on the toner carrier to the image carrier. An object of the present invention is to provide an image forming apparatus.

  In order to achieve the above object, the present invention has a toner carrier that is disposed opposite to an image carrier, carries toner and supplies the toner to the image carrier, and develops an electrostatic latent image formed on the surface of the image carrier. In an image forming apparatus comprising a developing device capable of executing a refresh process for discharging toner from the toner carrier side to the image carrier side during non-image formation, a toner formed on the image carrier during the refresh step The ejection pattern is characterized in that a line image having an acute angle with respect to the main scanning direction is ejected a plurality of times at a predetermined interval in the circumferential direction of the image carrier over the entire width of the development region.

  In the image forming apparatus having the above configuration, the line image has a line width of 2 mm or less.

  In the image forming apparatus having the above-described configuration, the angle of the line image with respect to the main scanning direction is 10 ° or more and 60 ° or less.

  According to the present invention, the image forming apparatus having the above-described configuration includes a control unit that controls a discharge length in the sub-scanning direction of the toner discharge pattern when the refresh process is executed based on a print image printing rate. .

  According to the present invention, in the image forming apparatus configured as described above, the linear velocity of the toner carrier during the refreshing process is made lower than that during image formation.

  According to the first configuration of the present invention, since the toner discharge pattern is configured by an obliquely inclined line image, it is difficult for the toner discharge amount to vary in both the axial direction and the circumferential direction of the toner carrier. The toner on the toner carrier can be discharged more uniformly. In addition, since the toner discharge pattern is composed of line images, it is possible to efficiently discharge the small-diameter overcharged toner by edge enhancement, and also suppress the toner adhesion to the transfer roller as compared with the case where the toner discharge pattern is a solid image. be able to.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the line width of the line image constituting the toner ejection pattern is set to 2 mm or less, whereby the edge portion in the toner ejection pattern is obtained. Therefore, the small-diameter overcharged toner can be more efficiently removed from the toner carrier by edge enhancement.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the angle of the line image with respect to the main scanning direction is set to 10 ° or more and 60 ° or less, whereby the image carrier. Since the toner adhesion amount per unit area increases, the toner on the toner carrier can be more efficiently ejected and the refresh effect can be enhanced.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the sub-scanning direction of the toner discharge pattern when the refresh process is executed based on the printing rate of the print image By controlling the discharge length of the toner, the amount of toner shortage corresponding to the actual printing rate is calculated and the discharge length of the toner discharge pattern is determined. It can be.

  According to the fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, the linear velocity of the toner carrying member during the refresh process is decreased as compared with the time of image formation. In addition, not only the toner on the surface layer on the toner carrier but also the toner in the lower layer can be discharged to the image carrier side, and the surface of the toner carrier can be effectively refreshed.

1 is a schematic cross-sectional view showing the overall configuration of an image forming apparatus of the present invention. Sectional drawing of the developing device used for the image forming apparatus of this invention 1 is a block diagram showing a control path of an image forming apparatus of the present invention. Timing chart showing an example of ON / OFF timing of charging bias, laser exposure, developing bias, and transfer bias in the refresh process The figure which shows an example of the toner discharge pattern used for this invention The figure which shows the toner discharge pattern whose line image is perpendicular | vertical to the main scanning direction The figure which shows the toner discharge pattern whose line image is parallel to the main scanning direction 7 is a flowchart showing an execution procedure of a refresh process in the image forming apparatus of the present invention. The graph which shows the relationship between the line width of the line image which comprises a toner discharge pattern, and the toner adhesion amount on a drum The graph which shows the relationship between the line width of the line image which comprises a toner discharge pattern, and the large diameter toner adhesion amount on a drum The graph which shows the relationship between the line width of the line image which comprises a toner discharge pattern, and the small amount toner adhesion amount on a drum The graph which shows the relationship between the angle with respect to the main scanning direction of the line image which comprises a toner discharge pattern, and the toner adhesion amount on a drum

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram showing the overall configuration of the image forming apparatus of the present invention, and the right side is shown as the front side of the image forming apparatus. As shown in FIG. 1, a paper feed cassette 2 that accommodates stacked paper is disposed at the lower part of the main body of the image forming apparatus 100. Above this paper feed cassette 2 is formed a paper transport path 4 that extends substantially horizontally from the front of the main body to the rear of the main body and further extends upward to reach the paper discharge section 3 formed on the upper surface of the main body. A pickup roller 5, a feed roller 6, an intermediate conveyance roller 7, a registration roller pair 8, an image forming unit 9, a fixing unit 10, and a discharge roller pair 11 are arranged in this order from the upstream side along the conveyance path.

  The paper feed cassette 2 is provided with a paper stacking plate 12 that is rotatably supported with respect to the paper feed cassette 2 by a rotation fulcrum 12a provided at the rear end in the paper transport direction. The paper stacked on the paper 12 is pressed against the pickup roller 5. Further, a retard roller 13 is disposed in front of the paper feed cassette 2 so as to be in pressure contact with the feed roller 6. When a plurality of sheets are simultaneously fed by the pickup roller 5, these feed rollers 6 are fed. A sheet is rolled by the roller 6 and the retard roller 13, and only the uppermost sheet is conveyed.

  Then, the sheet fed by the feed roller 6 and the retard roller 13 is conveyed to the registration roller pair 8 by changing the conveyance direction to the rear of the apparatus by the intermediate conveyance roller 7, and the timing is adjusted by the registration roller pair 8. Are supplied to the image forming unit 9.

  The image forming unit 9 forms a predetermined toner image on a sheet by an electrophotographic process. The photosensitive drum 14 is an image carrier that is pivotally supported in a clockwise direction in FIG. Above the charging device 15, the developing device 16, the cleaning device 17, the transfer roller 18 disposed so as to face the photosensitive drum 14 across the paper conveyance path 4, and the photosensitive drum 14. An exposure unit (LSU) 19 is arranged, and a toner container 20 for supplying toner to the developing device 16 is arranged above the developing device 16.

  The charging device 15 is provided with a conductive rubber roller 15 a connected to a power source (not shown), and the conductive rubber roller 15 a is disposed so as to contact the photosensitive drum 14. Then, when the photosensitive drum 14 rotates, the conductive rubber roller 15a comes into contact with the surface of the photosensitive drum 14 and is driven to rotate. At this time, a predetermined voltage is applied to the conductive rubber roller 15a, whereby the photosensitive drum 14 is rotated. The surface of the film is uniformly charged.

  Next, an electrostatic latent image based on the image data input on the photosensitive drum 14 is formed by the laser beam from the exposure unit (LSU) 19, and toner is attached to the electrostatic latent image by the developing device 16, and the photosensitive member. A toner image is formed on the surface of the drum 14. Then, a sheet is supplied from the registration roller pair 8 to a nip portion (transfer position) between the photosensitive drum 14 and the transfer roller 18 at a predetermined timing, and a toner image on the surface of the photosensitive drum 14 is formed on the sheet by the transfer roller 18. Transcribed.

  The sheet on which the toner image has been transferred is separated from the photosensitive drum 14 and conveyed toward the fixing unit 10. The fixing unit 10 is disposed on the downstream side of the image forming unit 9 in the sheet conveyance direction, and the sheet on which the toner image is transferred in the image forming unit 9 includes a heating roller 21 provided in the fixing unit 10 and the heating roller 21. The toner image heated and pressed by the pressure roller 22 pressed against the heating roller 21 is fixed on the toner image transferred to the paper.

  The paper on which the image is formed in the image forming unit 9 and the fixing unit 10 is discharged to the paper discharge unit 3 by the discharge roller pair 11. On the other hand, the toner remaining on the surface of the photosensitive drum 14 after the transfer is removed by the cleaning device 17. The photosensitive drum 14 is charged again by the charging device 15 and image formation is performed in the same manner. Further, an external temperature / humidity sensor 23 is installed above the toner container 20 for the purpose of measuring the device installation environment (machine peripheral environment).

  FIG. 2 is a side sectional view of the developing device mounted on the image forming apparatus of the present invention. As shown in FIG. 2, the developing device 16 includes a casing 31a that stores a one-component developer made of magnetic toner, and a cover 31b that seals the toner stored in the casing 31a so that the toner does not leak outside. A first spiral member 32, a second spiral member 33, a developing roller 35, and a regulating blade 36 are provided in the casing 31 that is configured.

  The inside of the casing 31a is partitioned into a first storage chamber 38 and a second storage chamber 39 by a partition plate 37 extending in the longitudinal direction (the paper surface direction in the figure). The member 32 and the second spiral member 33 are disposed in the second storage chamber 39, respectively. Further, the partition plate 37 is not provided at both ends in the longitudinal direction of the casing 31a, and this portion is a passage (developer delivery portion) through which the toner moves between the first storage chamber 38 and the second storage chamber 39. It has become.

  The 1st spiral member 32 and the 2nd spiral member 33 are comprised by the rotating shafts 32a and 33a and the spiral wings 32b and 33b arrange | positioned along the outer peripheral surface of the rotating shafts 32a and 33a, and are mutually substantially parallel. It is rotatably supported in the casing 31a. The first spiral member 32 and the second spiral member 33 are configured to rotate and convey the toner in the first storage chamber 38 and the second storage chamber 39 by rotating in a predetermined direction. In addition, the cover 31b is supplied with toner from a toner supply port (see FIG. 1) so that toner can be supplied into the casing 31a according to the detection result of a toner amount detection sensor (not shown). (Not shown) is provided.

  The developing roller 35 is rotatably supported in the first storage chamber 38 so as to be substantially parallel to the first spiral member 32 and the second spiral member 33. A fixed magnet body 40 made of a permanent magnet having a plurality of magnetic poles (here, three poles) is fixed inside the developing roller 35. When the developing roller 35 rotates according to the rotation of the photosensitive drum 14, the developing roller 35 rotates. Toner adheres (carries) to the surface of the developing roller 35 by the magnetic force of the fixed magnet body 40 to form a toner layer. The toner adhering to the developing roller 35 is caused by the potential difference between the surface potential of the photosensitive drum 14 and the developing bias applied to the developing roller 35 at the developing nip where the developing roller 35 and the photosensitive drum 14 face each other. It flies to the drum 14 and adheres to the photosensitive layer, and a toner image is formed on the surface of the photosensitive drum 14.

  The regulating blade 36 regulates the amount of toner supplied to the photosensitive drum 14, that is, the amount of toner adhering to the developing roller 35. For example, a magnetic material such as SUS (stainless steel) is used. The regulating blade 36 is disposed so that a predetermined gap (0.2 to 0.3 mm) is formed between the tip of the regulating blade 36 and the developing roller 35. The amount of toner adhering to the developing roller 35 is regulated by the magnetic field generated in the gap and the gap, and a thin toner layer of several tens of microns is formed on the surface of the developing roller 35.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. Further, the ROM 92 (or RAM 93) stores a reference print rate used for calculating the number of printed sheets for determining whether or not the refresh process needs to be performed and the print length (toner discharge length) of the toner discharge pattern, and the rotation of the photosensitive drum 1. The speed etc. are stored. The counter 95 adds up the number of printed sheets and counts it. Note that the number of times may be stored in the RAM 93, for example, without providing the counter 95 separately.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. The parts and devices controlled by the control unit 90 include, for example, the paper feed cassette 2, the fixing unit 10, the developing device 16, the exposure unit 19, the toner container 20, the external temperature / humidity sensor 23, the image input unit 50, and bias control. Examples include the circuit 51 and the operation unit 60.

  When the image forming apparatus 100 is a printer as shown in FIG. 1, the image input unit 50 is a receiving unit that receives image data transmitted from a personal computer or the like. When the image forming apparatus 100 is a copying machine, Scanning optical system equipped with a scanner lamp that illuminates the document during copying and a mirror that changes the optical path of the reflected light from the document, a condensing lens that focuses the reflected light from the document and forms an image It is an image reading unit composed of a CCD or the like that converts image light into an electrical signal. The image signal input from the image input unit 50 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 51 is connected to the charging bias power source 52, the developing bias power source 53, and the transfer bias power source 54, and operates each of these power sources by an output signal from the control unit 90. These power sources are bias controlled. A predetermined bias is applied to the charging device 15, the developing roller 35, and the transfer roller 18 in accordance with a control signal from the circuit 51.

  The operation unit 60 is provided with a liquid crystal display unit 61, LEDs 62 indicating various states, and a numeric keypad 63. The user operates the operation unit 60 to input instructions, thereby making various settings of the image forming apparatus 100. And execute various functions such as image formation. The liquid crystal display unit 61 displays the state of the image forming apparatus 100, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. The numeric keypad 63 is used for setting the number of copies to be printed and for inputting the fax number of the other party when the image forming apparatus 100 also has a fax function.

  In addition, the operation unit 60 includes a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, and various settings of the image forming apparatus 100 are set to a default state. A reset button or the like is provided for use.

  When the image forming apparatus of the present invention is not transferred to a recording medium, for example, when the image forming apparatus is started up from a power-off state or a sleep (power saving) mode to a copy start state, or a predetermined number of prints are performed. Sometimes, a refreshing process for discharging the toner on the developing roller 35 in the developing device 16 to the photosensitive drum 14 side can be executed.

  FIG. 4 is an example of a timing chart showing ON / OFF timings of the charging bias, laser exposure, developing bias, and transfer bias (cleaning bias) in the refresh process, and FIG. 5 shows toner formed on the photosensitive drum. It is a figure which shows an example of a discharge pattern. The refresh procedure execution procedure will be described with reference to FIGS. 4 and 5 with reference to FIGS. Here, a case where positively charged toner is used is described.

  When the execution of the refresh process is instructed, as shown in FIG. 4, the charging bias applied to the charging device 15 is turned on, and the surface of the photosensitive drum 14 is uniformly positively charged. At the same time, the developing bias having the same polarity (positive) as the toner applied to the developing roller 35 is also turned ON. Next, laser exposure by the exposure unit 19 is repeatedly turned ON / OFF a predetermined number of times during t1, and the charge on the surface of the photosensitive drum 14 is partially attenuated.

  When this laser exposure area (charge attenuation area) comes to a position facing the developing roller 35 by the rotation of the photosensitive drum 14, the toner on the developing roller 35 repels the developing bias and adheres to the laser exposure area. The ON state of the developing bias is continued until the photosensitive drum 14 rotates by a predetermined amount and the laser exposure area completely passes the developing roller 35, and is turned OFF after t2 after the charging bias and the laser exposure are turned OFF. Become. Then, a toner discharge pattern P as shown in FIG. 5 is formed on the photosensitive drum 14.

  FIG. 5 is a diagram showing an example of a toner discharge pattern used in the present invention, and shows a state in which the photosensitive drum 14 is developed on a plane. In the toner discharge pattern P, a line image L having a predetermined inclination with respect to the main scanning direction (left-right direction in the figure) is repeatedly developed at predetermined intervals over the width H of the developing area of the developing roller 35 (see FIG. 2). 4 is determined by the discharge length X of the toner discharge pattern P in the drum circumferential direction (sub-scanning direction). A method for determining the discharge length X will be described later.

  It is known that when the toner discharge pattern is composed of a large number of line images, the amount of deteriorated toner attached to the photosensitive drum 14 is increased as compared with a solid image. This is because the edge-enhancing force increases the toner attracting force, and the charged-up toner from the developing roller 35 to the photosensitive drum 14 can be transferred efficiently.

  When attention is paid to the relationship between the line width of the line image constituting the toner discharge pattern and the toner particle size adhering to the drum surface, the toner particle size decreases as the line width decreases. This is because, as the line width becomes narrower, the ratio of the edge area to the toner discharge pattern increases, and the small diameter toner having a high charge amount existing on the outermost surface of the developing roller 35 is easily developed to the edge area having a large potential difference. . Accordingly, the refreshed effect can be further enhanced by preferentially discharging the charged-up small-diameter toner that adversely affects the development performance to the photosensitive drum 14 side.

  The present invention is characterized in that the toner discharge pattern P is composed of an oblique line image L. Here, a toner discharge pattern P1 composed of a line image L1 perpendicular to the main scanning direction as shown in FIG. 6 and a toner discharge pattern composed of a line image L2 parallel to the main scanning direction as shown in FIG. Compared to P2, the toner discharge pattern P has a larger toner adhesion amount (toner discharge amount) per unit area than the toner discharge patterns P1 and P2. Although the reason for this has not been clarified yet, the following reasons are conceivable.

  In the toner discharge pattern P1, the toner discharge amount varies in the axial direction (main scanning direction) of the developing roller 35 between the portion where the line image L1 exists and the portion where the line image L1 does not exist (between lines). A region where no ink is discharged is generated. For the same reason, the toner discharge amount in the circumferential direction (sub-scanning direction) of the developing roller 35 is likely to vary in the toner discharge pattern P2.

  On the other hand, since the line image L constituting the toner discharge pattern P has an angle with respect to the main scanning direction, the toner discharge amount is less likely to vary in both the axial direction and the circumferential direction of the developing roller 35. Therefore, the toner discharge pattern P can discharge the toner on the developing roller 35 more uniformly, and even if the patterns have the same area represented by X × H, the toner adhesion amount per unit area Is estimated to increase. In addition, there may be a difference in the state of the electrostatic latent image potential due to the inclination of the line image and the influence of the development system.

  In order to refresh the entire circumference of the developing roller 35, the length X of the toner discharge pattern P in the drum circumferential direction (sub-scanning direction) needs to be longer than the outer circumferential length of the developing roller 35. is there. Further, the angle, line width, and line interval of the line image L may be set as appropriate according to the specifications of the image forming apparatus. However, in order to efficiently remove the overcharged toner, the angle of the line image L is set to 10 °. It is preferable that the angle is 60 ° or less and the line width is 2 mm or less. Further, the line interval may be set to such an extent that the toner on the developing roller 35 can be discharged without unevenness.

  Thereafter, the toner discharge pattern P passes through the nip portion between the photosensitive drum 14 and the transfer roller 18 as the photosensitive drum 14 rotates, and is finally collected from the drum surface by the cleaning device 17. A transfer bias (transfer reverse bias) having the same polarity (positive) as that of the toner is applied to the transfer roller 18 in order to prevent the toner discharge pattern P from adhering. The ON state of the reverse transfer bias is continued until the photosensitive drum 14 rotates by a predetermined amount and the toner discharge pattern P completely passes through the transfer roller 18, and is turned OFF with a delay of t3 after the developing bias is turned OFF. .

  At this time, since the toner discharge pattern P is pressed against the transfer roller 18, a part of the toner discharge pattern P physically adheres to the transfer roller 18 regardless of the application of the transfer reverse bias. The toner attached to the transfer roller 18 includes not only positively charged toner but also reversely charged (negatively charged) toner.

  Accordingly, a cleaning bias (transfer positive bias) having a reverse polarity (negative) to the toner is applied to the transfer roller 18, and the charging bias is turned ON again to positively charge the drum surface. As a result, the reversely charged toner adhering to the transfer roller 18 repels the negative cleaning bias, and is attracted to the positive potential on the drum surface and moves to the photosensitive drum 14 side. At this time, the charging bias ON time is equal to the cleaning bias application time t4 having a polarity opposite to that of the toner, and the charging bias ON timing is t6 earlier than the timing of applying the cleaning bias having the polarity opposite to that of the toner.

  Subsequently, a cleaning bias (transfer reverse bias) having the same polarity (positive) as that of the toner is applied to the transfer roller 18 and the charging bias is turned off again. As a result, the positively charged toner adhering to the transfer roller 18 is repelled by the positive cleaning bias, and is attracted to the drum surface (0 V) and moves to the photosensitive drum 14 side. At this time, the charging bias OFF time is equal to the cleaning bias application time t5 having the same polarity as the toner, and the charging bias OFF timing is earlier by t6 than the cleaning bias application timing having the same polarity as the toner.

  In FIG. 4, a cleaning bias having the opposite polarity and the same polarity as the toner is applied once, and the charging bias is turned on and off once in the same cycle as one cycle, and this is repeated for two cycles. The positively charged toner and the reversely charged toner on the roller 18 are removed. In order to clean the entire outer peripheral surface of the transfer roller 18, a cleaning bias application time (charging bias ON time) t4 having a polarity opposite to that of the toner and a cleaning bias application time (charging bias OFF time) having the same polarity as that of the toner are used. It is preferable that t5 be equal to or longer than the time required for the transfer roller 18 to make one rotation. Here, in the present invention, since the toner discharge pattern P is constituted by the line image L, the toner adhesion to the transfer roller 18 can be suppressed as compared with the case of the solid image.

  Next, a method for determining the discharge length X of the toner discharge pattern P will be described. A discharge length setting table that defines the discharge length X for each print rate level is used in order to set an optimal toner discharge amount according to the print rate of the image to be printed regardless of the usage status of the user. The discharge length setting table is stored in the ROM 92 (or RAM 93).

  An example of the discharge length setting table is shown in Table 1. In Table 1, the average printing rate is ranked into a plurality of levels (here, 5 levels), and the discharge length (mm / sheet) per image corresponding to each printing rate level is assigned. For example, assuming that the number of prints for executing the refresh process is A, the control unit 90 calculates the print rate bn for each image based on the digital signal in the temporary storage unit 94, and further integrates the print rate bn. Is calculated. Then, the integrated printing rate Σbn is divided by the number of printed sheets A counted by the counter 95 to calculate an average printing rate B (%) per printed sheet A. The discharge length per one image corresponding to the calculated average printing rate B is read from the discharge length setting table, and this is multiplied by the number of printed sheets A until the refresh process is executed, so that the discharge length X (mm) at the time of executing the refresh process is obtained. Calculated.

  Further, the discharge length X may be obtained by calculation. When the reference printing rate (the printing rate threshold that requires refreshing) is C (%), the printing rate difference C−B between the reference printing rate C and the above-described average printing rate B is the required discharge amount per image. Since (consumption shortage) is multiplied by the number of printed sheets A, (CB) × A (%) becomes a toner discharge amount W (%) necessary for executing the refresh process. The discharge length X is calculated by X = W / W1, where W1 is the toner discharge amount (% / mm) per unit length of the toner discharge pattern P. Note that the number A of printed sheets and the reference print rate C (%) can be appropriately set according to the use state of the user, the characteristics of the toner, the measured value of the external temperature and humidity sensor 23, and the like.

  Incidentally, in order to increase the image density at the time of image formation, the developing roller 35 and the photosensitive drum 14 are normally set to have a higher linear speed of the developing roller 35 than that of the photosensitive drum 14 at the time of image formation. The amount of toner per unit time supplied to the proximity range (development nip portion) is increased. Therefore, when the toner is forcibly discharged while the developing roller 35 is rotated at the same linear velocity as that at the time of image formation, only the toner that is easily developed on the surface layer among the thin toner layers formed on the developing roller 35 is on the photosensitive drum 14 side. Since the toner in the lower layer remains on the developing roller 35, a sufficient refresh effect cannot be obtained.

  Therefore, it is preferable to set the rotation speed (linear speed) of the developing roller 35 during execution of the refresh process to be slower than during image formation. By setting in this way, the time during which the electric field between the drum and the development acts on the toner thin layer on the developing roller 35 passing through the developing nip R (see FIG. 2) becomes longer, and the toner is formed on the developing roller 35. In addition to the surface toner that can be easily developed in the thin toner layer, it is possible to discharge the toner in the lower layer, so that a sufficient refresh effect can be obtained without extending the execution time of the refresh process or applying a high-voltage development bias. Can do.

  FIG. 8 is a flowchart showing an example of the refresh process executed in the image forming apparatus of the present invention. The execution procedure of the refresh process will be described according to the steps of FIG. 8 with reference to FIGS. 1 to 5 as necessary.

  First, when the image forming process is started by the user operating the operation unit 60 or a personal computer, the counter 95 counts the number of printed sheets n (step S1). The control unit 90 calculates a printing rate bn for each image based on the digital signal in the temporary storage unit 94, and further calculates an integrated printing rate Σbn obtained by integrating the printing rate bn. Next, it is determined whether or not the number n of printed sheets has reached the predetermined number A (step S2). When n = A, the control unit 90 calculates the average printing rate B by Σbn / A (step S3).

  Next, the control unit 90 determines whether or not the calculated average printing rate B is lower than the reference printing rate (here, 1.0%) (step S4). When the average printing rate B is lower than 1.0%, the toner discharge amount (CB) is obtained by multiplying the printing rate difference C-B between the average printing rate B and the reference printing rate C (%) by the number of printed sheets A. XA (%) is calculated, and the discharge length X of the toner discharge pattern P is calculated (step S5).

  Then, a refresh process is executed according to the timing chart of FIG. 4, and the toner discharge pattern P is discharged onto the photosensitive drum 14 (step S6). After the toner discharge is completed, a cleaning bias is applied to the transfer roller 18 to clean the transfer roller 18 (step S7). After the refresh process is completed, the count number n of the counter 95 is reset to 0 (step S8) and the process returns to step S1 again. On the other hand, if the average printing rate B is 1.0% or more in step S4, the refresh process is not executed, the count number n of the counter 95 is reset to 0 (step S8), and the process returns to step S1 again.

  According to the above control, it is possible to shorten the execution time of the refresh process by efficiently removing the small diameter charged-up toner on the developing roller 35 in the refresh process and suppressing the toner adhesion to the transfer roller 18. It becomes a highly convenient image forming apparatus capable of suppressing wasteful toner consumption other than the printing operation. In addition, since the amount of insufficient toner corresponding to the actual printing rate is calculated using the average printing rate B of the printed image, and the discharge length X of the toner discharge pattern P is determined based on this, the refresh process is executed. The toner discharge amount at that time can be set to an optimum amount.

  In the above embodiment, the ejection length X is obtained by calculation using the number of printed sheets A, the average printing rate B, and the reference printing rate C. However, the ejection length is set using the ejection length setting table shown in Table 1. X may be determined.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, the average printing rate is calculated by the control unit 90, but an arithmetic unit that calculates the average printing rate may be provided separately from the control unit 90. The present invention is not limited to the monochrome printer as shown in FIG. 1, but various images having a developing device such as a monochrome copying machine, a tandem type or rotary type color copying machine, a color printer, a facsimile or a laser printer. Of course, it can be applied to a forming apparatus.

  Using the image forming apparatus of the present invention as shown in FIG. 1, the relationship between the line width of the line image constituting the toner discharge pattern shown in FIG. 5, the amount of toner adhering to the photosensitive drum, and the toner particle size Was investigated. The test machine conditions were a peripheral speed (system linear speed) of the photosensitive drum of 306 mm / sec, a drum surface potential of 270 V on the white background potential (dark potential), and an image portion potential (bright potential) of 20 V. As the developer, a positively charged magnetic toner having an average particle diameter of 5.0 to 9.0 μm was used.

  Further, the DC voltage (Vdc) applied to the developing roller was 175 V, the Vpp of the AC voltage (Vac) was 1.7 kV, the frequency was 2.8 kHz, and Duty = 57%. The results are shown in FIGS.

  As is clear from FIG. 9, it was confirmed that the amount of toner adhering to the drum increases as the line width of the line image becomes narrower, and the amount of adhesion increases remarkably especially when the line width is 2 mm or less. As apparent from FIGS. 10 and 11, as the line width of the line image becomes narrower, the ratio of the large diameter toner (> 10.8 μm) adhering to the drum decreases, and the small diameter toner (<4.3 μm). It was confirmed that the ratio of increased.

  The reason for this is that as the line width of the line image becomes narrower, the ratio of the edge area to the solid area increases, which is advantageous for developing small-diameter overcharged toner that is easily developed into an edge area having a large potential difference. It is done.

  The relationship between the angle of the line image constituting the toner discharge pattern and the amount of toner adhering to the photosensitive drum was investigated. The conditions of the test machine were the same as those in Example 1, and a square toner discharge pattern in which 33 lines having a line width of 0.3 mm and a length of 25 mm were drawn at equal intervals (about 0.5 mm) to a width of 25 mm was subjected to main scanning. The toner was adhered by rotating it by a predetermined angle with respect to the direction, and the amount of toner adhered per unit area was measured. The results are shown in FIG.

As apparent from FIG. 12, when the angle of the line image constituting the toner discharge pattern with respect to the main scanning direction is increased from 0 ° (horizontal), the toner adhesion amount gradually increases from 6.5 g / m ^2. It was 7.0 g / m ² around 45 °. Further, when the angle of the line image exceeded 45 °, the toner adhesion amount gradually decreased to 6.0 g / m ² (minimum value) at 90 °. In addition, by setting the angle of the line image to 10 ° or more and 60 ° or less, the toner adhesion amount per unit area becomes 6.8 to 7.0 g / m ² , and it is confirmed that the toner on the developing roller can be efficiently discharged. It was done.

  The present invention can be used for an image forming apparatus provided with a refresh process for refreshing toner on a toner carrier during non-image formation. By using the present invention, it is possible to efficiently discharge the overcharged toner on the toner carrying member without increasing the toner discharge amount, thereby enhancing the refresh effect, and also suppressing the back contamination of the recording medium due to the toner adhering to the transfer roller. Therefore, it is possible to provide an image forming apparatus capable of forming a high-quality image without causing problems such as density reduction and image fogging.

9 Image forming unit 14 Photosensitive drum 16 Developing device 35 Developing roller (toner carrier)
36 Control blade 40 Fixed magnet body 40a Main pole 90 Control unit (control means)
91 CPU
92 ROM
93 RAM
94 Temporary storage section 95 Counter 100 Image forming apparatus R Development nip section P Toner discharge pattern L Line image

Claims (5)

A developing device for developing an electrostatic latent image formed on the surface of the image carrier, the toner carrier having a toner carrier arranged to face the image carrier and supplying the toner to the image carrier;
In an image forming apparatus capable of executing a refresh process for discharging toner from the toner carrier side to the image carrier side during non-image formation,
The toner discharge pattern formed on the image carrier during the refreshing process has a line image having an acute angle with respect to the main scanning direction, a plurality of times at predetermined intervals in the circumferential direction of the image carrier over the entire width of the development region. An image forming apparatus having a discharged configuration.   The image forming apparatus according to claim 1, wherein a line width of the line image is 2 mm or less.   The image forming apparatus according to claim 1, wherein an angle of the line image with respect to a main scanning direction is 10 ° or more and 60 ° or less.   The image according to any one of claims 1 to 3, further comprising control means for controlling a discharge length in the sub-scanning direction of the line image when the refresh process is executed based on a printing rate of the print image. Forming equipment.   5. The image forming apparatus according to claim 1, wherein a linear velocity of the toner carrier during the refreshing process is reduced as compared with that during image formation.

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